(1) Field of the Invention
The present invention relates to a paraboloidal reflecting mirror for a headlamp mounted on and used for vehicles such as automobiles, and particularly to a composite reflecting mirror having a plurality of reflecting surfaces.
(2) Description of the Prior Art
In a headlamp for an automobile, radiant diverging light rays emitted by a light source are reflected by a paraboloidal reflecting mirror to illuminate the road ahead, and the light rays may be adjusted in direction by a group of cuts of a front lens to obtain the desired distribution of rays. The correction of the direction of the light rays by the group of cuts of the front lens employs the principle of a prism. Particularly, in order to spread light to the left and right, a cylindrical cut that may be regarded as a continuous prism (a cut portion is in the form of a cylindrical concave curved surface) is effectively used. A lens piece a of the cut portion is shown in FIG. 17. In this lens piece a, a cylindrical cut b is formed in the inner surface, and parallel reflecting luminous fluxes c reflected by the paraboloidal reflecting mirror are spread to left and right when they pass to the outside through the cylindrical cut b. Recently the air resistance of the vehicle has been taken up as a problem, and a headlamp having a large inclination angle .theta. of the front lens which is subjected to air pressure has been required. If the lens piece a is inclined at a fixed angle (.theta..gtoreq.20.degree.), there occurs a phenomenon that the luminous fluxes projected on a screen d hang at the end from a horizontal line e (hereinafter referred to as the hanging phenomenon of the down-light), which is the characteristic of the cylindrical cut, thereby failing to provide a proper distribution of rays. Therefore, a large inclination angle may not be used. Accordingly, the spreading of the luminous fluxes by the cylindrical cuts of the front lens is of limited value.
A reflecting mirror for spreading luminous fluxes in a horizontal direction is well known as disclosed, for example, in Japanese patent publication No. 58-145002. (See FIG. 18, a front view, and FIG. 19, a sectional view.)
According to this well known reflecting mirror 10 (FIGS. 18 and 19), when a light source is placed in the vicinity of a focal point of a paraboloidal columnar reflecting mirror, the reflected rays are spread to the left and right, as shown in FIG. 20, and reflected light rays which are not substantially spread vertically are obtained. This is a flux distribution pattern formed when no lens is provided in front of the reflecting mirror. As will be apparent from this flux distribution pattern, the angle of spreading to the left and right is within 20.degree., and no hanging phenomenon occurs.
In the case of a headlamp for an automobile, a "pass-each other flux distribution", designed for the situation in which automobiles pass each other head on, is particularly important, and it is desirable that the light be spread to the left and right at an angle which is .+-.15.degree. in terms of a standard, and substantially .+-.30.degree.. Accordingly, in the well known reflecting mirror, in order to obtain the practical pass-each other flux distribution during use, the fluxes are corrected by the lens cut by way of the front lens 11 to form a proper pass-each other flux distribution or there is required a groove having a wide horizontal width as clearly described in the aforesaid Japanese patent publication No. 58-14502. It has been found however that the widening of the horizontal width of the groove inevitably reduces the number of reflecting portions, and as the result, the remote illuminance (which is the position at which the maximum illuminance is required in view of the flux distribution characteristic) is considerably reduced, thus failing to form a headlamp which can perform satisfactorily. This phenomenon possibly results from the system wherein reflected rays by grooves are superposed as the distribution of fluxes which are approximately symmetrical to the left and right about the front surface, that is, a curve wherein each focal distance F of a paraboloidal column contacts a tangent line at each apex thereof is made to comprise the same paraboloidal line as the focal distance F, whereby the flux distribution characteristic does not provide the so-called core.